Queensland: City under the sea

Fish of many different species - and human visitors - meet at the Lighthouse Bommie on the Great Barrier Reef. Photo / Supplied

I roll backwards off the pontoon boat and sink into the ocean depths. Fresh air and blue sky are left behind as I descend into a silent world to meet whatever lies beneath.

Settling on the fine white sand at 23m, I'm within sight of a kaleidoscopic coral reef outcrop known as the Lighthouse Bommie, named because it is opposite the old lighthouse on Lady Elliot Island, the southernmost island on the Great Barrier Reef.

I'm torn between conflicting emotions. Being suspended in a weightless state in crystal-clear, tepid water, gives a comforting back-to-the-womb feeling. But knowing that giant marine mammals are lurking in this vicinity brings a heightened state of anxiety.

I'm conscious of a high-pitched sound, the hauntingly beautiful song of a humpback whale somewhere out in the ocean. I catch a glimpse of a sleek, menacing silhouette at the periphery of vision - it's a white-tip reef shark.

When a giant manta ray arrives my heart leaps, as a great white blanket of underbelly passes straight overhead. The 4m-wide monster hovers over the bommie, where it's instantly besieged by an army of hyperactive cleaner fish, the barbers and groomers of the reef.

In this undersea metropolis, the Lighthouse Bommie is Grand Central Station, the foremost fish-cleaning base on the reef. The manta ray hovers like an inter-galactic space ship while the assorted nibblers remove every trace of dead skin and parasites from his mouth and gills. The stealth bomber of the seas then makes a sweeping turn and vanishes into the gloom.

Witnessing this cameo of life beneath the waves makes me realise that inner space is a parallel universe as diverse and colourful as our own. The 1500 fish species on the Great Barrier Reef, the whales, dolphins, manta rays, dugongs and the 4000 species of molluscs live their lives in an ordered way with the same degree of connectedness and symbiotic relationships that we enjoy.

The submarine city certainly has all the colour and variety of an earthly metropolis. The scroungers, suckers, pickers, biters, scavengers and devourers inhabit a coral jungle. Some are timid and take shelter in the nooks and crannies of the reef; I particularly love the impish little Nemo of Hollywood fame.

Like their human counterparts, these undersea creatures are adaptable and resilient. The parrot fish sleeps in a cocoon of spun mucus, which hides its tell-tale smell from predators.

The cuttle fish changes its colour and texture at will and has two eyeballs in each eye to focus on close and distant objects simultaneously. Survival in the deep requires super ingenuity and a large dose of luck.

Around the Lighthouse Bommie there are bright yellow angelfish with sweeping tails, flighty butterfly fish, cheeky-beak parrot fish that dart up to my mask in a mock attack and, my favourite, the gorgeously attired comedians known as harlequin tuskfish.

It's easy to lose yourself in this watery realm as the senses become fully engaged with the marine life.

A shimmering mass of big-eye trevally, cautious and alert, are drifting slowly down the current navigating the passage like a convoy of VIP limos. Maori wrasse with intricate moko-like markings on their face, lie motionless like metro line commuters waiting for their train.

When the full moon lights up the ocean in September each year, the city stages a reproduction festival even more colourful than a Hero Parade.

Entire reefs of coral polyps eject a multicoloured shower of egg and sperm clusters in an upside-down snowstorm that drifts far and wide - carnival time for whale sharks and other plankton feeders. Returning to the pontoon boat I feel exhilarated ... and hungry. Laid out before us is a smorgasbord lunch of fresh fish, mussels, scallops, salads and tropical fruit. Our dive group is emulating the hungry citizens of that colourful city beneath the waves.